Self-adjusting optimal waveforms

ABSTRACT

An exemplary method includes detecting fibrillation, measuring impedance of a defibrillation circuit that includes myocardial tissue, determining one or more defibrillation shock parameters based at least in part on the impedance, delivering a defibrillation shock using the one or more defibrillation shock parameters and, if the shock was unsuccessful, adjusting a membrane time constant and determining one or more new defibrillation shock parameters based at least in part on the adjusted membrane time constant. Various other exemplary methods are disclosed as well as various exemplary devices, systems, etc.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 11/102,331, filed Apr. 7, 2005, titled “Self Adjusting OptimalWaveforms”; and is related to U.S. patent application Ser. No.11/102,002, filed Apr. 7, 2005, titled “Self Adjusting OptimalWaveforms,” now U.S. Pat. No. 7,283,871.

TECHNICAL FIELD

Subject matter presented herein generally relates to implantabledefibrillation devices. Various exemplary methods, devices, systems,etc., concern selection or determination of one or more defibrillationshock parameters.

BACKGROUND

Implantable cardiac defibrillators (ICDs) perform two main functions:detecting fibrillation and delivering defibrillation shocks. A varietyof issues are associated with use of ICDs. Some issues pertain to thepatient while others pertain to the ICD. For example, an ICD shouldextend patient life and even improve quality of life. On the other hand,an ICD should operate efficiently to conserve its limited power supply.

Efficient operation of an ICD involves delivering defibrillation shocksonly when required, delivering an initial defibrillation shock that hasa high likelihood of success, and delivering defibrillation shocks atenergy levels that are not greatly in excess of a minimum requiredenergy level. The first factor depends largely on fibrillation detectionalgorithms and ICD capabilities related thereto while the second andthird operational factors are interrelated.

Many studies have tried to divine “optimal” shock parameters. Forexample, a study by Inrich “How to program pulse duration or tilt inimplantable cardioverter defibrillators,” Pacing Clin Electrophysiol.2003 January; 26(1 Pt 2): 453-6, presented a system of three relatedequations in an effort to determine optimal defibrillation shockparameters. While such studies are instructive, a need still exists forbetter methods to determine or optimize defibrillation shock parameters.Yet further, as described herein, judicious selection of parameters ormodels or analysis of defibrillation shock information can even yieldinsight as to cardiac condition.

SUMMARY

An exemplary method includes detecting fibrillation, measuring impedanceof a defibrillation circuit that includes myocardial tissue, determiningone or more defibrillation shock parameters based at least in part onthe impedance, delivering a defibrillation shock using the one or moredefibrillation shock parameters and, if the shock was unsuccessful,adjusting a membrane time constant and determining one or more newdefibrillation shock parameters based at least in part on the adjustedmembrane time constant. Various other exemplary methods are disclosed aswell as various exemplary devices, systems, etc.

In general, the various methods, devices, systems, etc., describedherein, and equivalents thereof, are optionally suitable for use in avariety of pacing therapies and other cardiac related therapies.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the described implementations can be morereadily understood by reference to the following description taken inconjunction with the accompanying drawings.

FIG. 1 is a simplified diagram illustrating an exemplary implantablestimulation device in electrical communication with at least three leadsimplanted into a patient's heart and at least one other lead fordelivering stimulation and/or shock therapy. Other examples may includea different number of leads (e.g., fewer or more).

FIG. 2 is a functional block diagram of an exemplary implantablestimulation device illustrating basic elements that are configured toprovide cardioversion, defibrillation, pacing stimulation and/or othertissue and/or nerve stimulation. The implantable stimulation device isfurther configured to sense information and administer stimulationpulses responsive to such information.

FIG. 3 is a plot of potential versus time for a monophasicdefibrillation shock and membrane response thereto and a plot ofpotential versus time for a biphasic defibrillation shock and membraneresponse thereto.

FIG. 4 is a series of plots as taken from a study by Cheng et al. wheremembrane time constant varies with respect to several factors.

FIG. 5 is a plot of potential versus time for a truncated exponentialwaveform and a damped sine waveform and a plot of energy required fordefibrillation using such waveforms.

FIG. 6 is a plot of normalized membrane potential versus time forvarious membrane time constants and a plot of duration to the heart(membrane) to reach a certain normalized membrane potential given amembrane time constant.

FIG. 7 is a diagram of an exemplary method that aims to defibrillate theheart where measuring occurs at least prior to delivering a shock.

FIG. 8 is a diagram of an exemplary method that aims to defibrillate theheart where measuring occurs during a delivering period of a shock.

FIG. 9 is a diagram of an exemplary determination block that maydetermine one or more defibrillation shock parameters.

FIG. 10 is a block diagram of an exemplary method that aims todefibrillate the heart based at least in part on an estimated membranetime constant.

FIG. 11 is a block diagram of an exemplary method that aims todefibrillate the heart and store information pertaining to membrane timeconstant.

FIG. 12 is a block diagram of an exemplary method that decides ifischemia is present and then selects a membrane time constant based onthe decision, which may aid in determining one or more defibrillationshock parameters.

FIG. 13 is a plot of an exemplary ischemia indicator plotted asnormalized membrane time constant versus time in months.

DETAILED DESCRIPTION

The following description includes the best mode presently contemplatedfor practicing the described implementations. This description is not tobe taken in a limiting sense, but rather is made merely for the purposeof describing the general principles of the implementations. The scopeof the described implementations should be ascertained with reference tothe issued claims. In the description that follows, like numerals orreference designators will be used to reference like parts or elementsthroughout.

Exemplary Stimulation Device

The techniques described below are optionally implemented in connectionwith any stimulation device that is configured or configurable tostimulate and/or shock tissue.

FIG. 1 shows an exemplary stimulation device 100 in electricalcommunication with a patient's heart 102 by way of three leads 104, 106,108, suitable for delivering multi-chamber stimulation and shocktherapy. The leads 104, 106, 108 are optionally configurable fordelivery of stimulation pulses suitable for stimulation of autonomicnerves, non-myocardial tissue, other nerves, etc. In addition, thedevice 100 includes a fourth lead 110 having, in this implementation,three electrodes 144, 144′, 144″ suitable for stimulation of autonomicnerves, non-myocardial tissue, other nerves, etc. For example, this leadmay be positioned in and/or near a patient's heart or near an autonomicnerve within a patient's body and remote from the heart. The rightatrial lead 104, as the name implies, is positioned in and/or passesthrough a patient's right atrium. The right atrial lead 104 optionallysenses atrial cardiac signals and/or provide right atrial chamberstimulation therapy. As shown in FIG. 1, the stimulation device 100 iscoupled to an implantable right atrial lead 104 having, for example, anatrial tip electrode 120, which typically is implanted in the patient'sright atrial appendage. The lead 104, as shown in FIG. 1, also includesan atrial ring electrode 121. Of course, the lead 104 may have otherelectrodes as well. For example, the right atrial lead optionallyincludes a distal bifurcation having electrodes suitable for stimulationof autonomic nerves, non-myocardial tissue, other nerves, etc.

To sense atrial cardiac signals, ventricular cardiac signals and/or toprovide chamber pacing therapy, particularly on the left side of apatient's heart, the stimulation device 100 is coupled to a coronarysinus lead 106 designed for placement in the coronary sinus and/ortributary veins of the coronary sinus. Thus, the coronary sinus lead 106is optionally suitable for positioning at least one distal electrodeadjacent to the left ventricle and/or additional electrode(s) adjacentto the left atrium. In a normal heart, tributary veins of the coronarysinus include, but may not be limited to, the great cardiac vein, theleft marginal vein, the left posterior ventricular vein, the middlecardiac vein, and the small cardiac vein.

Accordingly, an exemplary coronary sinus lead 106 is optionally designedto receive atrial and ventricular cardiac signals and to deliver leftventricular pacing therapy using, for example, at least a leftventricular tip electrode 122, left atrial pacing therapy using at leasta left atrial ring electrode 124, and shocking therapy using at least aleft atrial coil electrode 126. For a complete description of a coronarysinus lead, the reader is directed to U.S. Pat. No. 5,466,254, “CoronarySinus Lead with Atrial Sensing Capability” (Helland), which areincorporated herein by reference. The coronary sinus lead 106 furtheroptionally includes electrodes for stimulation of autonomic nerves. Sucha lead may include pacing and autonomic nerve stimulation functionalityand may further include bifurcations or legs. For example, an exemplarycoronary sinus lead includes pacing electrodes capable of deliveringpacing pulses to a patient's left ventricle and at least one electrodecapable of stimulating an autonomic nerve. An exemplary coronary sinuslead (or left ventricular lead or left atrial lead) may also include atleast one electrode capable of stimulating an autonomic nerve,non-myocardial tissue, other nerves, etc., wherein such an electrode maybe positioned on the lead or a bifurcation or leg of the lead.

Stimulation device 100 is also shown in electrical communication withthe patient's heart 102 by way of an implantable right ventricular lead108 having, in this exemplary implementation, a right ventricular tipelectrode 128, a right ventricular ring electrode 130, a rightventricular (RV) coil electrode 132, and an SVC coil electrode 134.Typically, the right ventricular lead 108 is transvenously inserted intothe heart 102 to place the right ventricular tip electrode 128 in theright ventricular apex so that the RV coil electrode 132 will bepositioned in the right ventricle and the SVC coil electrode 134 will bepositioned in the superior vena cava. Accordingly, the right ventricularlead 108 is capable of sensing or receiving cardiac signals, anddelivering stimulation in the form of pacing and shock therapy to theright ventricle. An exemplary right ventricular lead may also include atleast one electrode capable of stimulating an autonomic nerve,non-myocardial tissue, other nerves, etc., wherein such an electrode maybe positioned on the lead or a bifurcation or leg of the lead.

FIG. 2 shows an exemplary, simplified block diagram depicting variouscomponents of stimulation device 100. The stimulation device 100 can becapable of treating both fast and slow arrhythmias with stimulationtherapy, including cardioversion, defibrillation, and pacingstimulation. The stimulation device can be solely or further capable ofdelivering stimuli to autonomic nerves, non-myocardial tissue, othernerves, etc. While a particular multi-chamber device is shown, it is tobe appreciated and understood that this is done for illustrationpurposes only. Thus, the techniques and methods described below can beimplemented in connection with any suitably configured or configurablestimulation device. Accordingly, one of skill in the art could readilyduplicate, eliminate, or disable the appropriate circuitry in anydesired combination to provide a device capable of treating theappropriate chamber(s) or regions of a patient's heart withcardioversion, defibrillation, pacing stimulation, autonomic nervestimulation, non-myocardial tissue stimulation, other nerve stimulation,etc.

Housing 200 for stimulation device 100 is often referred to as the“can”, “case” or “case electrode”, and may be programmably selected toact as the return electrode for all “unipolar” modes. Housing 200 mayfurther be used as a return electrode alone or in combination with oneor more of the coil electrodes 126, 132 and 134 for shocking purposes.Housing 200 further includes a connector (not shown) having a pluralityof terminals 201, 202, 204, 206, 208, 212, 214, 216, 218, 221 (shownschematically and, for convenience, the names of the electrodes to whichthey are connected are shown next to the terminals).

To achieve right atrial sensing and/or pacing, the connector includes atleast a right atrial tip terminal (A_(R) TIP) 202 adapted for connectionto the atrial tip electrode 120. A right atrial ring terminal (A_(R)RING) 201 is also shown, which is adapted for connection to the atrialring electrode 121. To achieve left chamber sensing, pacing and/orshocking, the connector includes at least a left ventricular tipterminal (V_(L) TIP) 204, a left atrial ring terminal (A_(L) RING) 206,and a left atrial shocking terminal (A_(L) COIL) 208, which are adaptedfor connection to the left ventricular tip electrode 122, the leftatrial ring electrode 124, and the left atrial coil electrode 126,respectively. Connection to suitable autonomic nerve stimulationelectrodes or other tissue stimulation or sensing electrodes is alsopossible via these and/or other terminals (e.g., via a nerve and/ortissue stimulation and/or sensing terminal S ELEC 221).

To support right chamber sensing, pacing, and/or shocking, the connectorfurther includes a right ventricular tip terminal (V_(R) TIP) 212, aright ventricular ring terminal (V_(R) RING) 214, a right ventricularshocking terminal (RV COIL) 216, and a superior vena cava shockingterminal (SVC COIL) 218, which are adapted for connection to the rightventricular tip electrode 128, right ventricular ring electrode 130, theRV coil electrode 132, and the SVC coil electrode 134, respectively.Connection to suitable autonomic nerve stimulation electrodes or othertissue stimulation or sensing electrodes is also possible via theseand/or other terminals (e.g., via a nerve and/or tissue stimulationand/or sensing terminal S ELEC 221).

At the core of the stimulation device 100 is a programmablemicrocontroller 220 that controls the various modes of stimulationtherapy. As is well known in the art, microcontroller 220 typicallyincludes a microprocessor, or equivalent control circuitry, designedspecifically for controlling the delivery of stimulation therapy, andmay further include RAM or ROM memory, logic and timing circuitry, statemachine circuitry, and I/O circuitry. Typically, microcontroller 220includes the ability to process or monitor input signals (data orinformation) as controlled by a program code stored in a designatedblock of memory. The type of microcontroller is not critical to thedescribed implementations. Rather, any suitable microcontroller 220 maybe used that carries out the functions described herein. The use ofmicroprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

Representative types of control circuitry that may be used in connectionwith the described embodiments can include the microprocessor-basedcontrol system of U.S. Pat. No. 4,940,052 (Mann et al.), thestate-machine of U.S. Pat. No. 4,712,555 (Thornander et al.) and U.S.Pat. No. 4,944,298 (Sholder), all of which are incorporated by referenceherein. For a more detailed description of the various timing intervalsused within the stimulation device and their inter-relationship, seeU.S. Pat. No. 4,788,980 (Mann et al.), also incorporated herein byreference.

FIG. 2 also shows an atrial pulse generator 222 and a ventricular pulsegenerator 224 that generate pacing stimulation pulses for delivery bythe right atrial lead 104, the coronary sinus lead 106, and/or the rightventricular lead 108 via an electrode configuration switch 226. It isunderstood that in order to provide stimulation therapy in each of thefour chambers of the heart the atrial and ventricular pulse generators,222 and 224, may include dedicated, independent pulse generators,multiplexed pulse generators, or shared pulse generators. The pulsegenerators 222 and 224 are controlled by the microcontroller 220 viaappropriate control signals 228 and 230, respectively, to trigger orinhibit the stimulation pulses.

Microcontroller 220 further includes timing control circuitry 232 tocontrol the timing of the stimulation pulses (e.g., pacing rate,atrio-ventricular (AV) delay, atrial interconduction (A-A) delay, orventricular interconduction (V-V) delay, etc.) as well as to keep trackof the timing of refractory periods, blanking intervals, noise detectionwindows, evoked response windows, alert intervals, marker channeltiming, etc., which is well known in the art.

Microcontroller 220 further includes an arrhythmia detector 234, amorphology discrimination module 236, a capture detection module 237, anauto sensitivity module 238, a membrane time constant module module 239and optionally an orthostatic compensator and a minute ventilation (MV)response module, the latter two are not shown in FIG. 2. Thesecomponents can be utilized by the stimulation device 100 for determiningdesirable times to administer various therapies, including those toreduce the effects of orthostatic hypotension. The aforementionedcomponents may be implemented in hardware as part of the microcontroller220, or as software/firmware instructions programmed into the device andexecuted on the microcontroller 220 during certain modes of operation.

The membrane time constant module 239 may perform a variety of tasksrelated to, for example, selection, determination, estimation, etc., ofa membrane time constant. Further, the module 239 may aid in ischemiadeterminations or decisions. For example, as described herein, amembrane time constant may indicate whether a patient has ischemia. Themodule 239 optionally relies on a variety of information, for example,the module 239 may rely on impedance measurements of a defibrillationshock circuit that includes tissue, fluid, etc., (generally myocardialtissue). The module 239 may aid in determining one or moredefibrillation shock parameters (e.g., energy, leading edge voltage,duration, phase, waveform type, timing, electrode configuration, etc.).In general, determining a parameter means determining a value, whetherthe value is an energy, a voltage, a duration, a number of phases, etc.For example, determining a leading edge voltage parameter meansdetermining a leading edge voltage suitable for use in delivery of ashock. The module 239 may operate prior to delivery of a shock or duringdelivery of a shock or during a shock delivery period (e.g., shockduration).

The electronic configuration switch 226 includes a plurality of switchesfor connecting the desired electrodes to the appropriate I/O circuits,thereby providing complete electrode programmability. Accordingly,switch 226, in response to a control signal 242 from the microcontroller220, determines the polarity of the stimulation pulses (e.g., unipolar,bipolar, combipolar, etc.) by selectively closing the appropriatecombination of switches (not shown) as is known in the art.

Atrial sensing circuits 244 and ventricular sensing circuits 246 mayalso be selectively coupled to the right atrial lead 104, coronary sinuslead 106, and the right ventricular lead 108, through the switch 226 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 244 and 246, may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. Switch 226determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity. The sensing circuits (e.g., 244 and 246) areoptionally capable of obtaining information indicative of tissuecapture.

Each sensing circuit 244 and 246 preferably employs one or more lowpower, precision amplifiers with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, as knownin the art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 100 to deal effectively withthe difficult problem of sensing the low amplitude signalcharacteristics of atrial or ventricular fibrillation.

The outputs of the atrial and ventricular sensing circuits 244 and 246are connected to the microcontroller 220, which, in turn, is able totrigger or inhibit the atrial and ventricular pulse generators 222 and224, respectively, in a demand fashion in response to the absence orpresence of cardiac activity in the appropriate chambers of the heart.Furthermore, as described herein, the microcontroller 220 is alsocapable of analyzing information output from the sensing circuits 244and 246 and/or the data acquisition system 252 to determine or detectwhether capture has occurred and to program a pulse, or pulses, inresponse to such determinations. The sensing circuits 244 and 246, inturn, receive control signals over signal lines 248 and 250 from themicrocontroller 220 for purposes of controlling the gain, threshold,polarization charge removal circuitry (not shown), and the timing of anyblocking circuitry (not shown) coupled to the inputs of the sensingcircuits, 244 and 246, as is known in the art.

For arrhythmia detection, the device 100 utilizes the atrial andventricular sensing circuits, 244 and 246, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. In reference toarrhythmias, as used herein, “sensing” is reserved for the noting of anelectrical signal or obtaining data (information), and “detection” isthe processing (analysis) of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the arrhythmia detector 234 of themicrocontroller 220 by comparing them to a predefined rate zone limit(i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillationrate zones) and various other characteristics (e.g., sudden onset,stability, physiologic sensors, and morphology, etc.) in order todetermine the type of remedial therapy that is needed (e.g., bradycardiapacing, anti-tachycardia pacing, cardioversion shocks or defibrillationshocks, collectively referred to as “tiered therapy”).

Cardiac signals are also applied to inputs of an analog-to-digital (A/D)data acquisition system 252. The data acquisition system 252 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device254. The data acquisition system 252 is coupled to the right atrial lead104, the coronary sinus lead 106, the right ventricular lead 108 and/orthe nerve or other tissue stimulation lead 110 through the switch 226 tosample cardiac signals across any pair of desired electrodes.

The microcontroller 220 is further coupled to a memory 260 by a suitabledata/address bus 262, wherein the programmable operating parameters usedby the microcontroller 220 are stored and modified, as required, inorder to customize the operation of the stimulation device 100 to suitthe needs of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape, number of pulses, and vector of eachshocking pulse to be delivered to the patient's heart 102 within eachrespective tier of therapy. One feature of the described embodiments isthe ability to sense and store a relatively large amount of data (e.g.,from the data acquisition system 252), which data may then be used forsubsequent analysis to guide the programming of the device.

Advantageously, the operating parameters of the implantable device 100may be non-invasively programmed into the memory 260 through a telemetrycircuit 264 in telemetric communication via communication link 266 withthe external device 254, such as a programmer, transtelephonictransceiver, or a diagnostic system analyzer. The microcontroller 220activates the telemetry circuit 264 with a control signal 268. Thetelemetry circuit 264 advantageously allows intracardiac electrogramsand status information relating to the operation of the device 100 (ascontained in the microcontroller 220 or memory 260) to be sent to theexternal device 254 through an established communication link 266.

The stimulation device 100 can further include a physiologic sensor 270,commonly referred to as a “rate-responsive” sensor because it istypically used to adjust pacing stimulation rate according to theexercise state of the patient. However, the physiological sensor 270 mayfurther be used to detect changes in cardiac output (see, e.g., U.S.Pat. No. 6,314,323, entitled “Heart stimulator determining cardiacoutput, by measuring the systolic pressure, for controlling thestimulation,” to Ekwall, issued Nov. 6, 2001, which discusses a pressuresensor adapted to sense pressure in a right ventricle and to generate anelectrical pressure signal corresponding to the sensed pressure, anintegrator supplied with the pressure signal which integrates thepressure signal between a start time and a stop time to produce anintegration result that corresponds to cardiac output), changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states). Accordingly, themicrocontroller 220 responds by adjusting the various pacing parameters(such as rate, AV Delay, V-V Delay, etc.) at which the atrial andventricular pulse generators, 222 and 224, generate stimulation pulses.

While shown as being included within the stimulation device 100, it isto be understood that the physiologic sensor 270 may also be external tothe stimulation device 100, yet still be implanted within or carried bythe patient. Examples of physiologic sensors that may be implemented indevice 100 include known sensors that, for example, sense pressure,respiration rate, pH of blood, ventricular gradient, cardiac output,preload, afterload, contractility, and so forth. Another sensor that maybe used is one that detects activity variance, wherein an activitysensor is monitored diurnally to detect the low variance in themeasurement corresponding to the sleep state. For a complete descriptionof the activity variance sensor, the reader is directed to U.S. Pat. No.5,476,483 (Bornzin et al.), issued Dec. 19, 1995, which patent is herebyincorporated by reference.

More specifically, the physiological sensors 270 optionally includesensors for detecting movement and minute ventilation in the patient.The physiological sensors 270 may include a position sensor and/or aminute ventilation (MV) sensor to sense minute ventilation, which isdefined as the total volume of air that moves in and out of a patient'slungs in a minute. Signals generated by the position sensor and MVsensor are passed to the microcontroller 220 for analysis in determiningwhether to adjust the pacing rate, etc. The microcontroller 220 monitorsthe signals for indications of the patient's position and activitystatus, such as whether the patient is climbing upstairs or descendingdownstairs or whether the patient is sitting up after lying down.

The stimulation device 100 additionally includes a battery 276 thatprovides operating power to all of the circuits shown in FIG. 2. For thestimulation device 100, which employs shocking therapy, the battery 276is capable of operating at low current drains for long periods of time(e.g., preferably less than 10 μA), and is capable of providinghigh-current pulses (for capacitor charging) when the patient requires ashock pulse (e.g., preferably, in excess of 2 A, at voltages above 200V, for periods of 10 seconds or more). The battery 276 also desirablyhas a predictable discharge characteristic so that elective replacementtime can be detected.

The stimulation device 100 can further include magnet detectioncircuitry (not shown), coupled to the microcontroller 220, to detectwhen a magnet is placed over the stimulation device 100. A magnet may beused by a clinician to perform various test functions of the stimulationdevice 100 and/or to signal the microcontroller 220 that the externalprogrammer 254 is in place to receive or transmit data to themicrocontroller 220 through the telemetry circuits 264. Trigger IEGMstorage also can be achieved by magnet.

The stimulation device 100 further includes an impedance measuringcircuit 278 that is enabled by the microcontroller 220 via a controlsignal 280. The known uses for an impedance measuring circuit 278include, but are not limited to, lead impedance surveillance during theacute and chronic phases for proper lead positioning or dislodgement;detecting operable electrodes and automatically switching to an operablepair if dislodgement occurs; measuring respiration or minuteventilation; measuring thoracic impedance for determining shockthresholds (HF indications—pulmonary edema and other factors); detectingwhen the device has been implanted; measuring stroke volume; anddetecting the opening of heart valves, etc. As already mentioned, thecircuit 278 may provide impedance information to the membrane timeconstant module 239. The impedance measuring circuit 278 isadvantageously coupled to the switch 226 so that any desired electrodemay be used.

In the case where the stimulation device 100 is intended to operate asan implantable cardioverter/defibrillator (ICD) device, it detects theoccurrence of an arrhythmia, and automatically applies an appropriatetherapy to the heart aimed at terminating the detected arrhythmia. Tothis end, the microcontroller 220 further controls a shocking circuit282 by way of a control signal 284. The shocking circuit 282 generatesshocking pulses in a range of joules, for example, conventionally up toabout 40 J, as controlled by the microcontroller 220. Such shockingpulses are applied to the patient's heart 102 through at least twoshocking electrodes, and as shown in this embodiment, selected from theleft atrial coil electrode 126, the RV coil electrode 132, and/or theSVC coil electrode 134. As noted above, the housing 200 may act as anactive electrode in combination with the RV electrode 132, or as part ofa split electrical vector using the SVC coil electrode 134 or the leftatrial coil electrode 126 (i.e., using the RV electrode as a commonelectrode).

Cardioversion level shocks are generally considered to be of low tomoderate energy level (so as to minimize pain felt by the patient),and/or synchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range ofapproximately 5 J to approximately 40 J), delivered asynchronously(since R-waves may be too disorganized), and pertaining exclusively tothe treatment of fibrillation. Accordingly, the microcontroller 220 iscapable of controlling the synchronous or asynchronous delivery of theshocking pulses.

In low-energy cardioversion, an ICD device typically delivers acardioversion stimulus (e.g., 0.1 J, etc.) synchronously with a QRScomplex; thus, avoiding the vulnerable period of the T wave and avoidingan increased risk of initiation of VF. In general, if antitachycardiapacing or cardioversion fails to terminate a tachycardia, then, forexample, after a programmed time interval or if the tachycardiaaccelerates, the ICD device initiates defibrillation therapy.

While an ICD device may reserve defibrillation as a latter tier therapy,it may use defibrillation as a first-tier therapy for VF. In general, anICD device does not synchronize defibrillation therapy with any givenportion of an ECG. Again, defibrillation therapy typically involveshigh-energy shocks (e.g., 5 J to 40 J), which can include monophasic orunidirectional and/or biphasic or bidirectional shock waveforms.Defibrillation may also include delivery of pulses over two or morecurrent pathways.

FIG. 3 shows a plot of potential versus time 310 for a defibrillationshock. More specifically, the plot 310 shows a monophasic, truncatedexponential waveform 314 and a corresponding myocardial or membraneresponse 316. The waveform or “pulse” 314 may be characterized in partby a parameter called “tilt” and a duration parameter. Tilt is definedas the leading edge voltage at or near an initial time, which istypically the maximum voltage, minus the voltage at the end of theduration divided by the leading edge voltage. Thus, for the example inthe plot 310, the waveform has a tilt of approximately 65% while theduration of the shock waveform is about 7 ms. Monophasic, truncatedexponential waveforms may be characterized by leading edge voltage, tiltand duration. The leading edge voltage typically depends on resistanceor impedance of the tissue into which the shock is delivered and thecapacitor(s) used to store charge. Many have described this relationshipusing an RC circuit, noting that a given tilt and RC circuit parametersmay determine the duration, that a given duration and RC parameters maydetermine tilt, etc.

During delivery of the shock, the membrane is charged and its potentialincreases. Various optical and electrode mapping studies have revealedthe shape of the membrane, which may be characterized by a membrane timeconstant (τ) according to the following equation:

V(t)/V _(max)=(1−e ^(−t/τ))  (1)

where V(t) is the potential with respect to time t and V_(max) is themaximum potential prior to activation. Such studies indicate that, inhumans, the time constant in healthy adults is about 3.5 ms (note thatthe time constant in the plot is considerably shorter). Models otherthan that of Eqn. 1 may be used for membrane time constant.

A study by Gold et al., “Strength-Duration Relationship for HumanTransvenous Defibrillation,” Circulation. 1997; 96: 3517-3520, notedthat the strength-duration relationship for human transvenousdefibrillation is unaffected by pulse widths greater than 6 ms and thatthese data were consistent with a parallel circuit model (which differsfrom Eqn. 1) having a membrane time constant of 5.3 ms. Gold et al.further state that delivered energy is minimized at 5 to 6 ms,“indicating that this is the most efficient pulse widths for monophasicdefibrillation and should serve as the basis for the first phase ofbiphasic waveforms.”

In the plot 310, the data correspond to a capacitor charged to 100% ofits voltage and then discharged to deliver the monophasic shock. Thecell membrane potential increases during delivery of the shock andreaches a peak at about 4 ms. However, the shock duration (i.e.,duration of the monophasic waveform) is greater than the time requiredby the membrane to reach the peak. Thus, energy is wasted and themembrane subjected to more energy than necessary to achieve the peakpotential. Further, the extended duration of the shock iscounterproductive as it reduces the final membrane response.

FIG. 3 also shows a plot of potential versus time 320 for anotherdefibrillation shock. More specifically, the plot 320 shows a biphasic,truncated exponential waveform 324 and a corresponding myocardial ormembrane response 326. Biphasic, truncated exponential waveforms may becharacterized in part by a duration of a first phase and a duration of asecond phase. In general, an overall tilt (T) may be given, for example,the waveform 324 has an overall tilt of about 50%(T=1−V_(trailing)/V_(leading)).

During delivery of the shock, the membrane is charged and its potentialincreases. However, in contrast to the monophasic shock, the secondphase of the biphasic shock commences at about the peak (e.g.,approximately 3.5 ms) and acts to decrease the membrane potential.

If during the first phase, the cell is captured, a new activationpotential results and the second phase achieves little. If the cell isonly marginally charged during the first phase, then the second phaseacts to remove the charge and thereby bring the cell to a baselinelevel. If the cell is electroporated then the second phase, by quicklyremoving the excess charge sitting on the membrane, acts to quickly“heal” the cell.

In the example of the plot 320, the duration of the second phase, at 3.5ms, is too long and the membrane is actually discharged and takenslightly negative. This is a suboptimal result noting that a secondphase duration of about 2.5 ms would have been optimal.

FIG. 4 shows three plots of time constant (t) versus voltage for normaland ischemic conditions in rabbits where shocks were delivered at 25%(plot 410), 50% (plot 420) and 75% (plot 430) of the action potentialduration (APD). These data are from a study by Cheng et al., “Mechanismsof shock-induced arrhythmogenesis during acute global ischemia,” Am JPhysiol Heart Circ Physiol 282: H2141-H2151, 2002. This study inducedglobal acute cardiac ischemia by rapid reduction of the flow rate by 75%where a relatively steady state of action potential duration (APD) wasreached between 20 and 30 minutes of the flow reduction. In ischemic andnormal conditions, truncated exponential, monophasic shocks of 8 ms induration were delivered from a 150-pF capacitor defibrillator (HVS-02,Ventritex) between two electrodes (apex and right ventricle). To improvethe fidelity of time constant measurement, Cheng et al. used only strongtransmembrane responses to the shock (amplitude >10 mV) and traces witha signal-to-noise ratio above 75.

The data in the plots 410, 420 and 430 indicate that, for a truncatedexponential, monophasic shock waveform (with positive voltage), timeconstant of the myocardium decreases with increasing shock voltage,increases with increasing fraction of APD delivery and with ischemia.Result for a negative voltage, i.e., hyperpolarization, differed or wereinconclusive for ischemia (particularly at 50% APD) and for fraction ofAPD (time constant increased with respect to fraction of APD) whiletrends with respect to voltage were maintained.

FIG. 5 shows a plot 510 of potential versus time for a truncatedexponential waveform 514 and a Gurvich waveform 518 and a plot 520 ofdefibrillation energy for a truncated exponential waveform 514 and aGurvich waveform 518. A study by Qu et al., “The Gurvich defibrillationwaveform has lower defibrillation threshold than the Zoll waveform andthe truncated exponential waveform in the rabbit heart,” Can. J.Physiol. Phar. 2004 (in press) indicates that the Gurvich waveform(damped sine wave) has a lower threshold than the Zoll or the truncatedexponential waveforms. However, another study (Bardy et al., “TruncatedBiphasic Pulses for Transthoracic Defibrillation,” Circulation. 1995;91:1768-1774) states that damped sine wave pulses may offer no advantagefor ICDs and would result in an increase in ICD size and weight becauseof the need to generate higher voltages compared to other waveforms.Regardless of this opinion, the type of shock waveform may be animportant parameter in determining an optimal shock, recognizing thatcertain devices may be limited in waveform capabilities.

A study by Mowrey et al., “Kinetics of defibrillation shock-inducedresponse: design implications for the optimal defibrillation waveform,”Europace. 2002 January; 4(1):27-39, concluded that the time constant ofthe membrane depends on the field, action potential phase and the shockpolarity and suggested use of a slower shock leading edge, since themembrane cannot follow potentially damaging faster waveforms. Timeconstants were based on a single exponential model and data for rabbitsyielded time constants in a range from 1.6 ms to 14.2 ms.

FIG. 6 shows a plot 610 of normalized potential versus time and a plot620 of duration for reaching a particular fraction of maximum potentialversus time constant. According to the plot 610, ischemia causes anincrease in time constant. According to the plot 620, an increase intime constant due to ischemia corresponds to a longer duration for themyocardium to reach a desired potential (e.g., 99%, 99.9%, etc. of amaximum potential).

FIG. 7 shows an exemplary method 700 for delivering a defibrillationshock. A detection block 704 detects a need for a shock. While thedetection block 704 indicates detection of ventricular fibrillation(VF), other types of arrhythmia may be treated using one or more shocks.A measurement block 708 follows that measures lead impedance. If arecent lead impedance measurement has been made or lead impedance isotherwise known, then the exemplary method 700 may proceed to asubsequent block. The exemplary method 700 follows in a determinationblock 712 that determines one or more shock parameters based any of avariety of factors. A delivery block 716 delivers the shock according tothe one or more parameters. The delivery block 716 may call for deliveryof more than one shock as the one or more shock parameters may pertainto more than one shock.

The exemplary method 700 may operate as follows: the detection block 704may detect ventricular fibrillation; the measurement block 708 maymeasure impedance of a defibrillation circuit that includes myocardialtissue; the determination block 712 may determine one or moredefibrillation shock parameters based at least in part on the impedance;and the delivery block 716 may deliver a defibrillation shock using theone or more defibrillation shock parameters. Further, if the shock wasunsuccessful, the exemplary method may adjust a membrane time constantand then determine one or more new defibrillation shock parameters basedat least in part on the adjusted membrane time constant. Such anexemplary method may relate an increase in the membrane time constant topresence of ischemia or worsening of ischemia. In the case the shock wassuccessful, one or more parameters may be stored or analyzed to yield amembrane time constant.

FIG. 8 shows an exemplary method 800 for delivering a defibrillationshock that includes measuring during delivery of the shock and anexemplary plot 830 of voltage versus time where a parameter (e.g.,duration) depends at least in part on the measuring. A detection block804 detects a need for a shock. While the detection block 804 indicatesdetection of ventricular fibrillation (VF), other types of arrhythmiamay be treated using one or more shocks. A commencement block 808follows that commences delivery of a defibrillation shock. A measurementblock 812 follows that measures lead impedance. For example, for abiphasic shock, the first phase generally exceeds 3 ms. Thus, in thisexample, about 1 ms after commencement of the shock, measurement ofvoltage and current may occur. Impedance may then be approximated by thevoltage divided by the current.

A determination block 816 may rely on this impedance (or othermeasurement, etc.) may be used to adjust one or more shock parameterssuch as shock duration. An adjustment block 820 adjusts one or moreshock parameters, if required. An adjustment can include terminating theshock. For a biphasic shock, consider the plot 830, where an on-the-flymeasurement (e.g., at Δt₁) or measurements may be used to determine oradjust shock duration of the first phase (t₁), the second phase (t₂) oran overall duration (e.g., t₁+t₂). Of course, other parameters may beadjusted in addition to or alternative to shock duration. Furthermeasurements, determinations and adjustments may occur, as representedby the dashed line that loops from the adjustment block 820 to themeasurement block 812. Various exemplary methods optionally includemeasuring prior to commencement of a shock and during commencement of ashock.

Various exemplary methods may make determinations or adjustments usingadditional information, which may be information measured in vivo,information acquired via wireless communication, etc.

As mentioned for the exemplary method 700, if the shock wasunsuccessful, the exemplary method 800 may adjust a membrane timeconstant and then determine one or more new defibrillation shockparameters based at least in part on the adjusted membrane timeconstant. Such an exemplary method may relate an increase in themembrane time constant to presence of ischemia or worsening of ischemia.In the case the shock was successful, one or more parameters may bestored or analyzed to yield a membrane time constant.

Various exemplary methods optionally rely on selection of a membranetime constant and determining one or more shock parameters based on theselected constant, which may be implemented in a determination blocksuch as the block 712 or the block 816. If a shock is unsuccessful,rather than adjusting the one or more shock specific parameters (e.g.,duration, voltage, tilt, etc.), the membrane time constant is adjustedupward or downward. In response, one or more new shock parameters aredetermined based on the adjusted membrane time constant. In suchexemplary methods, the membrane time constant becomes an importantfactor in defibrillation therapy. Further, actual measurement of themembrane time constant may not be required. Instead, a model relates oneor more shock parameters to a membrane time constant. In other exemplarymethods, the membrane time constant may be used to determine only asingle shock parameter, for example, shock duration. In yet otherexemplary methods, leading edge voltage and membrane time constant maybe used to determine shock duration. Of course, membrane time constantcan be used in a variety of manners in determining one or more shockparameters.

Various exemplary methods optionally rely on changes to one or moreshock parameters to estimate a membrane time constant. This approach maybe referred to as an inverse approach. In such exemplary methods,information stored by an ICD may be analyzed to determine a membranetime constant or a change in membrane time constant over time. Thevalues may then be related to ischemia (see, e.g., the plots of FIG. 4).

An exemplary method that aims to defibrillate the heart may usedefibrillation waveforms based on cardiac membrane response to one ormore shocks where adjustments may occur to an assumed membrane timeconstant in response to an unsuccessful shock. An exemplary method thataims to defibrillate the heart may detect ischemia and in response tothe detection, adjust duration of a defibrillation shock, typically tolengthen the duration.

Various exemplary methods optionally rely on ischemia detection by anyof a variety of techniques (e.g., sensors, parameter changes, etc.) andthen use such information to select an appropriate membrane timeconstant for use in determining one or more shock parameters. Forexample, as shown in the plots of FIG. 4, if ischemia is present, thenmembrane time constant has increased and may increase further if theischemia progresses.

FIG. 9 shows an exemplary determination block 912, which may serve asthe determination block 712 of the exemplary method 700 or thedetermination block 816 of the exemplary method 800. The determinationblock 912 includes various sub-blocks 921-929. More specifically, thelisted sub-blocks include sub-blocks for impedance 921, history 922,ischemia 923, voltage/current 924, electrode configuration 925, timing926, waveform 927, medication 928 and other 929. An exemplarydetermination block may rely on one or more of such sub-blocks. Thesub-blocks 921-929 may represent inputs or outputs. Inputs are generallyvarious factors that may be used to determine one or more outputparameters. Various features of the exemplary determination block 912may be included in an exemplary implantable device, such as, but notlimited to, the exemplary device 100 of FIGS. 1 and 2 (see, e.g., themodule 239, etc.).

With respect to impedance 921, a study by Inrich “How to program pulseduration or tilt in implantable cardioverter defibrillators,” PacingClin Electrophysiol. 2003 January; 26(1 Pt 2):453-6, presented a systemof three related equations: a tilt equation, a pulse duration equationand an RC time constant equation, where “R” is the output resistance orimpedance and “C” is the output capacitance. Inrich based the RC timeconstant on knowledge of the pulse generator's output capacitance andthe lead system's resistance. With this value and an assumed chronaxietime (the pulse duration at which twice the rheobase current is requiredto evoke a threshold response), Inrich presented a manner to obtain “theoptimal values to which the PD and tilt should be programmed.” Inrichfurther suggested a “realistic range” for the RC time constant fromabout 2.5 ms to about 9 ms and a relationship where tilt decreases overthis range from about 65% to about 45%.

With respect to chronaxie time constant and membrane time constant, astudy by Swerdlow et al. “Application of Models of Defibrillation toHuman Defibrillation Data: Implications for Optimizing ImplantableDefibrillator Capacitance,” Circulation, 1997; 96: 2813-2822, presentedmodel time constants and predicted optimal biphasic waveforms forepicardial and transvenous deliveries. Such information may be used indetermining one or more shock parameters.

The history sub-block 922 may introduce information pertaining to pastsuccess or number of prior shocks when determining a shock parameter.The ischemia sub-block 923 may introduce ischemia information that isoptionally related to trends reported in the aforementioned study byCheng et al. The voltage/current sub-block 924 may introduce informationas to voltage, current, energy, etc., of past shocks or limitations ofan implantable device. The electrode configuration sub-block 925 mayintroduce information pertaining to location or type of an electrode orelectrodes. As already mentioned, epicardial and transvenous shocks mayuse different shock parameters. The timing sub-block 926 may introduceinformation pertaining to the timing of a shock with respect to, forexample, an action potential. The waveform sub-block 927 may introduceinformation related to type of waveform and phase. The medicationsub-block 928 may introduce information as to medication taken by apatient that could have an effect on shock therapy or risk offibrillation or other arrhythmia. The “other” sub-block 929 is includedas to cover various other factors that may be used to optimize one ormore shock parameters.

The exemplary determination block 912 may rely on one or more inputs todetermine or estimate a myocardial time constant. Once such a timeconstant has been determined, then a look-up table, a model, etc., maybe used to select one or more shock parameters, preferably parametersdeemed optimal for termination of ventricular fibrillation. Such adetermination may occur during delivery or prior to delivery of a shock.In either instance, upon delivery of a shock based on such parameters,success or failure may be noted (e.g., stored) or used in a feedbackloop to optimize defibrillation success. Such information may be usefulin diagnosing patient condition or future therapy.

The exemplary determination block 912 may include control logic forcontrolling actions of an exemplary implantable device. Such logic isoptionally implemented as instructions on one or more computer readablemedia that can, for example, enable a microprocessor to operateaccordingly. Referring again to the exemplary device 100 of FIGS. 1 and2, such a device may include logic for determinations or other actions.For example, an exemplary device may optionally include control logic todetect fibrillation, to determine one or more defibrillation shockparameters based at least in part on a measured impedance, to call fordelivery a defibrillation shock using a lead and the one or moredefibrillation shock parameters, to decide whether the shock wasunsuccessful and, if the shock was unsuccessful, to adjust a membranetime constant and to determine one or more new defibrillation shockparameters based at least in part on the adjusted membrane timeconstant.

In another example, an exemplary device may optionally include controllogic to detect fibrillation, to decide whether a patient has cardiacischemia, to select a defibrillation shock duration based at least inpart on the deciding, to deliver a defibrillation shock using a lead andthe selected defibrillation shock duration, to decide whether the shockwas unsuccessful and, if the shock was unsuccessful, to adjust thedefibrillation shock duration and to deliver another defibrillationshock using the adjusted defibrillation shock duration.

In yet another example, an exemplary device may optionally includecontrol logic to determine cardiac membrane time constants based atleast in part on stored defibrillation shock parameters and to decidewhether a patient has ischemia. In another example, an exemplary devicemay optionally include control logic to detect fibrillation, to commencedelivery of a defibrillation shock using a lead, to determine one ormore defibrillation shock parameters based at least in part on ameasured impedance and to adjust one or more defibrillation shockparameters during delivery of a defibrillation shock. Of course, otherexamples exist, which may rely on control logic to implement variousexemplary methods, such as, but not limited to, the exemplary methods700, 800, 1000, 1100, and 1200.

FIG. 10 shows an exemplary method 1000 for delivering a defibrillationshock. In a detection block 1004, detection of ventricular fibrillationoccurs. A measurement block 1008 follows that measures lead impedance. Aselection block 1012 selects a particular waveform or uses a defaultwaveform, for example, a biphasic truncated exponential (TE) waveform.Based on the impedance and selected waveform, another selection block1016 selects a voltage, for example, a leading edge voltage of adefibrillation shock. The exemplary method 1000 then estimates amyocardial time constant (e.g., a membrane time constant, τ) in anestimation block 1020, for example, based on historic information. Giventhe waveform type, the leading edge voltage and the estimated timeconstant, a determination block 1024 determines duration for a firstphase of the biphasic waveform. In general, the duration aims to avoidwaste of energy, i.e., to achieve capture without delivering excessivecharge to the myocardial tissue. A delivery block 1028 follows where ashock is delivered in an effort to defibrillate the heart.

Various exemplary methods optionally include a measurement block thatmeasures impedance after commencement of a shock. Based on the impedanceand selected waveform, a block may select a voltage, for example, basedoptionally in part on duration since commencement of the shock. Such anexemplary method may estimate a myocardial time constant (e.g., amembrane time constant, τ), for example, as in the exemplary method1000. On the basis of the measured information or other information, adetermination block may determine a duration for a first phase of thebiphasic waveform and an adjustment block may adjust the duration of thefirst phase during delivery of the shock. Of course, such an exemplarymethod may operate during a subsequent phase or during delivery of othertypes of waveforms.

FIG. 11 shows an exemplary method 1100 for defibrillation of the heart.The exemplary method 1100 commences in a detection block 1104 thatdetects ventricular fibrillation. A measurement block 1108 then measuresimpedance between two or more electrodes intended for delivery of adefibrillation shock to the heart. Based at least in part on themeasured impedance, the method 1100 adjusts a defibrillation shockwaveform assuming a membrane time constant τ=τ_(base), for example, ofapproximately 3.5 ms. The adjustment may act to adjust the duration ofthe waveform to minimize delivery of excessive energy to the myocardium.A delivery block 1116 follows that delivers a defibrillation shock usingthe adjusted waveform.

The exemplary method 1100 then continues with a decision block 1120 thatdecides whether the defibrillation shock was successful. If thedelivered shock was a success, then the method 1100 may continue at thedetection block 1104 where normal monitoring may occur. However, if thedelivered shock did not successfully defibrillate the heart, then themethod 1100 continues in another adjustment block 1124 that adjusts theshock energy upward, for example, to a maximum. In general, adefibrillation method should aim to defibrillate the heart in anexpedient manner. Thus, if an initial shock does not succeed, then thenext shock should typically have a greater probability of success, eventhough shock energy may be in excess of the optimal shock energy. Adelivery block 1128 delivers the shock at the higher energy, while stillrelying on the membrane time constant τ_(base).

Another decision block 1132 follows delivery of the higher energy shock.If the higher energy shock successfully defibrillated the heart, thenthe method 1100 continues at the detection block 1104. However, if thehigher energy shock does not succeed in defibrillating the heart, thenthe method 1100 continues in a waveform adjustment block 1136 thatrelies on a different membrane time constant, for example, according tothe equation: τ=τ_(base)−Δτ. A shock delivery block 1140 follows theadjustment block 1136.

Thus, to this point, the exemplary method 1100 has delivered two shocksusing a waveform based at least in part on a base membrane time constantand a third shock using a waveform based at least in part on an adjustedmembrane time constant. In general, the third shock is delivered using ahigh energy level, for example, a maximum energy. After delivery of thethird shock per the delivery block 1140, the method 1100 enters yetanother decision block 1144 that decides if the shock successfullydefibrillated the heart. If the shock was successful, then the method1100 stores the associated membrane time constant (e.g., the constant ofthe block 1136) in a storage block 1152 and may return to the detectionblock 1104. However, if the third shock does not succeed, then anotherdelivery block 1148 may follow and deliver one more shock. Similarly,another decision block 1156 may follow the delivery block 1148. In theinstance that the additional shock does not succeed in defibrillatingthe heart, then another adjustment may occur in an adjustment block 1164where the membrane time constant is adjusted according to the equation:τ=τ_(base)+Δτ.

A delivery block 1169 delivers a shock according to the adjustedmembrane constant and a decision block 1172 decides to store themembrane time constant via the storage block 1152 if the shocksuccessfully defibrillates the heart or it may issue an alert per thealert block 1180 if the shock does not succeed.

While the exemplary method 1100 includes a measurement block 1108 priorto the delivery block 1116, a measurement block may be implementedduring delivery of a shock, between phase switching or at other timesduring the overall shock duration. Such measured information may be usedto determine or adjust one or more shock parameters during delivery orprior to delivery of a subsequent shock (e.g., delivery block 1128,1140, 1169). A measurement block may be implemented during delivery,between phase switching or at other times during the overall shockduration of shocks associated with one or more of the delivery blocks1128, 1140 and 1169.

Various exemplary methods optionally rely on an initial membrane timeconstant and one or more adjustments to such a value to determine one ormore defibrillation shock parameters. For shocks that succeed indefibrillation, the membrane time constant may be stored or normalizedand stored.

FIG. 12 shows an exemplary method 1200 for delivering a defibrillationshock. According to the exemplary method 1200, a detection block 1204detects ventricular fibrillation. A decision block 1208 follows thatdecides if ischemia is present. Information for making such a decisionmay come from any of a variety of sensors or from an external source viawireless communication (e.g., transtelephonic communication, etc.). Ifthe decision block 1208 decides that ischemia is not present, then adetermination block 1212 determines one or more shock parameters using abase membrane time constant (τ=τ_(base)). However, if the decision block1208 decides that ischemia is present, then a determination block 1216determines one or more shock parameters using a membrane time constantthat is greater than the base time constant (e.g.,τ=τ_(base)+Δτ_(ischemia)). The membrane time constant in the case ofischemia is optionally selected based on degree of ischemia. Forexample, a Δτ_(ischemia) may vary depending on degree of ischemia.

FIG. 13 shows an exemplary ischemia indicator plot 1300 of membrane timeconstant, optionally normalized, versus time in months. The solid linerepresents successful shocks while the individual points representunsuccessful shocks. A dashed line represents an optional alert levelbased on membrane time constant exceeding a certain value. Shockfrequency for ICD patients can vary from less than once per month tomore than once per day. Defibrillation thresholds have been reported tovary with circadian, seasonal and other rhythms. Such variations may beexpected with respect to membrane time constant as well. However, asshown in FIG. 4, ischemia corresponds to an increase in membrane timeconstant. Further, ischemia may occur over a time-scale that exceedssuch other rhythms. Consequently, varying shock parameters on the basisof membrane time constant can provide useful information as to ischemia.

Various exemplary methods optionally deliver atrial shocks which maysimilarly rely on a membrane time constant. Further, atrial shocks maybe delivered more frequently than ventricular shocks, which, in turn,may allow for a better indication of ischemia.

Although exemplary methods, devices, systems, etc., have been describedat times in language specific to structural features or methodologicalacts, it is to be understood that the subject matter defined in theappended claims is not necessarily limited to the specific features oracts described. Rather, the specific features and acts are disclosed asexemplary forms of implementing various claimed subject matter.

1. A method comprising: detecting fibrillation; measuring impedance of adefibrillation circuit that includes myocardial tissue; determining oneor more defibrillation shock parameters based at least in part on theimpedance; delivering a defibrillation shock using the one or moredefibrillation shock parameters; and if the shock was unsuccessful,adjusting a membrane time constant and determining one or more newdefibrillation shock parameters based at least in part on the adjustedmembrane time constant.
 2. The method of claim 1 further comprisingrelating an increase in the membrane time constant to presence ofischemia or worsening of ischemia.
 3. The method of claim 1 furthercomprising storing membrane time constants associated with successfuldefibrillation shocks.
 4. The method of claim 1 wherein the determiningrelies in part on an RC circuit model.
 6. The method of claim 1 whereinthe defibrillation shock has a biphasic, truncated exponential waveform.7. The method of claim 1 wherein the one or more defibrillation shockparameters include leading edge voltage, tilt and shock duration.
 8. Themethod of claim 7 wherein the shock duration parameter corresponds toduration of a first phase of a biphasic waveform.
 9. The method of claim1 wherein the defibrillation shock has a positive voltage.
 10. A methodcomprising: providing a history of one or more defibrillation shockparameters for a patient fitted with an ICD; determining membrane timeconstants based at least in part on the history of defibrillation shockparameters; and deciding whether the patient has ischemia.
 11. Themethod of claim 10 wherein the history spans more than one month. 12.The method of claim 10 wherein the determining relies on a model. 13.The method of claim 10 wherein the one or more defibrillation shockparameters includes defibrillation shock duration.
 14. An implantableapparatus comprising: memory operative to store one or moredefibrillation shock parameters; and control logic to determine cardiacmembrane time constants based at least in part on stored defibrillationshock parameters and to decide whether the patient has ischemia.